The present invention refers to an improved method and apparatus for the large-scale production of containers made of a thermoplastic material, such as in particular polyethylene terephtalate (PET) and polypropylene (PP), intended for applications involving their being filled with liquids that may be also at quite elevated temperature and/or contain CO2 (carbon dioxide) gas.
In the field of technologies and machines for producing containers of the above cited kind there are a number of developments and improvements aimed at obtaining, on the one side, increasingly robust and improved containers, capable of being used for both hot-filled liquids and carbonated beverages, and, on the other side, production processes and related apparatuses that are capable of manufacturing said containers in an increasingly reliable, cost-effective, versatile manner to increasingly high quality standards in a highly competitive industrial context of very large-scale production.
Such production processes are generally known to be able to be schematically divided into two basic typologies, ie. single-stage and two-stage processes.
The features and characteristics that are typical of said processes, and differentiate them from each other in terms of both equipment and operation, are largely known to all those skilled in the art, so that no need arises here to dwell on them any longer.
Inherent to any single-stage process there is the fact that an uneven heat distribution is unfailingly brought about in the cross-sectional direction of the wall thickness of the preform when the latter is removed from the related cavity in the mold into which the molten resin had been injected.
Various processes have been actually patented, which cover time and temperature parameters of the preform when this is removed from the injection mold, in view of optimizing the related cycle times.
All patent literature covering single-stage processes discloses almost invariably a final forming or molding stage of the container of thermoplastic material which is in some way or other carried through a conditioning station to reach an even wall temperature throughout the cross-section of the same wall, said temperature corresponding to the preferred molecular orientation temperature of the thermoplastic resin involved.
A currently most preferred technique involves the use of a process of continuous extrusion of a flow of molten resin so as to sequentially fill a plurality of molds, such as this is described in the following patent specifications:
GB 767,164
FR 2.089.154
FR 2.114.455
U.S. Pat. No. 4,242,073.
Furthermore, owing to the difference existing between the time requirements for the injection molding/cool-down phase and the blow-molding phase, which usually requires a time that may be even four times shorter than the one involved in the previous phase, the practice is also largely known which consists in using, in a single-stage apparatus, a number of blow-molding molds which is a whole multiple of the number of corresponding injection-molding molds, so as to compensate in this way for the difference existing in the time requirements of the two phases. These phases, in fact, are unavoidably organized in series, with a greater number of injection-molding molds in such a manner as to produce performs in such quantities and at such a rate as to fully saturate the utilization of the blow-molding molds.
The patent specifications U.S. Pat. Nos. 4,261,949 and 4.313,720, both to Emhart Industries Inc., totally self-explaining and descriptive in this connection.
The above cited teachings, i.e. the continuous extrusion in a plurality of multiple molds, associated to a combination of blowing molds which are in a number that is a multiple of the number of the corresponding injection molds, have been brought together, and generally improved and illustrated, in the U.S. Pat. No. 4,372,910 and its divisional U.S. Pat. No. 4,470,796 to Stroup, while a preferred embodiment thereof has been implemented in a single-stage injection-blow molding apparatus of Van Dorn Plastic Machinery generally disclosed in said U.S. patent to Stroup.
Said U.S. Pat. No. 4,372,910 (along with its divisional) essentially describes and claims a process and apparatus for the production of hollow plastic products, typically bottles.
As compared with the prior-art, the therein disclosed invention covers a single-stage extrusion process for producing preforms that are successively stretched and blown into the final or finished product. Said extrusion process includes the possibility of carrying out a continuous extrusion, wherein the molten material is fed in sequence into a plurality of preform-molding molds. The process is organized in such a manner that, when the extruded melt is being fed into a mold, e.g. the first one of a set of molds, the other molds are closed by appropriate valves actuated in a sequence. Once said first mold has been filled, the same is closed, while a second mold is opened to be in turn filled by the flow of extruded melt. This sequence goes on until all of the molds in the set have been filled, after which the process is started again from the first mold that has in the meantime transferred its preforms to the respective blow-molding station.
The sequence in which the valves are actuated is such as to ensure that the molten material is absorbed, i.e. taken up almost constantly, so that the extrusion process takes place continuously, i.e. without interruptions, with clear operational and practical advantages.
One of the most significant advantages claimed in the above cited patent lies in the fact that, considering that the preform molding operation takes a considerably longer time, which quite often is a multiple of the time required by the subsequent blow-molding operation, it is nor necessary to wait, at the end of a blow-molding operation, for a subsequent preform molding operation to reach its conclusion (which thing would lead to a fully inadequate efficiency level of the blow-molding station), since through a multiplication in parallel and a process for cyclically feeding a plurality of preform molding molds which in turn are adapted to feed a plurality of blow-molding stations, a better utilization of the whole production plan is allegedly obtained.
Another advantage is claimed to lie in the fact that the continuous extrusion process contributes to a further improvement in the utilization of the plant, so that the combination of the various elements described in the above cited patent would conclusively lead to a drastic improvement in the overall production efficiency.
However, the invention disclosed in the above cited patent has in practice a number of drawbacks that tend to limit the extent of the claimed advantages: some of such drawbacks are described in the Italian patent no. 1 265 567, filed by the Applicant, to which reference is therefore made here for the sake of brevity. Anyway, they essentially relate to the problems that are typically brought about to the resin filling and compression operations performed by the same resin extruder, as well as the therewith associated problems brought about by the resin undergoing shear stress, i.e. frictioning, and hence generating acetaldehyde.
Further drawbacks, which generally come afloat in plants with a high output capacity, derive from the fact that an increase in the number of cavities in each mold, an increase in the number of the same molds and therefore an increase in the total volume and, hence, in the cost of the molds, along with an increase in the cost of the control apparatuses that have to be associated therewith, when added to an increase in the length and the diameter of the extruder and, as a result, in the related costs and production and logistic complexity, lead to overall charges that are no more sustainable, even in front of an altogether moderate increase in output capacity.
Furthermore, the greater total number of mold cavities provided in such plants require a correspondingly high flow of molten resin to be processed by the extruder and this of course requires the use of an extruder with a correspondingly large diameter. It has however repeatedly been found that, in such a kind of extruders, the film of resin that tends to spread out on the inner walls of the extrusion barrel, and fails to be directly pushed forward by the screw, actually picks up a no more negligeable thickness and, owing to the type of path to be covered when moving towards the cavities, it tends to particularly concentrate in some cavities, while neglecting other ones.
Considering now that such a resin moves forward at a slower rate and is in direct contact with the heating walls, it ensues that it is heated up with a greater intensity due to the combinded effect of the longer dwelling time in the extruder and said contact with the heating wall; such a greater and sensible overheating effect tends to generate a considerable amount of acetaldehyde that concentrates, along with said fraction of resin, just in said determined cavities.
In conclusion, preforms are in this way produced in which there is a concrete risk of releasing acetaldehyde in amounts that are clearly beyond the allowable limits set by the applying regulations.
From the disclosure in the European patent EP 0 071 258 to Valyi, the teaching is also known according to which the preform is allowed to rapidly cool down in the injection mold until its temperature sinks below the point to which the highest crystallization rate for the given plastic material corresponds. The same preform is then quickly removed from the injection mold while it still has an uneven distribution of its temperature, and transferred to a holding station provided between the injection mold and the conditioning station so as to allow the heat content of the preform to keep changing until an average temperature suitable for the orientation is reached, thereby setting the injection mold free for the next preform molding operation and, therefore, reaching a shorter cycle time while preserving the basic properties of the bottle to be produced.
Such a teaching, however, involves the addition of a supplementary holding station, i.e. mold with all associated drawbacks of an economic nature that this implies and all related production complications deriving from the construction of such a mold along with the therewith associated insertion, removal, ejection, driving and control means.
Based on the above considerations, it therefore is a main purpose of the present invention to provide a single-stage process and apparatus for the production on an industrial scale of a container of thermoplastic resin which is thermally stable, capable of being filled with both hot and carbonated liquids wherein a higher output is ensured with a smaller number of molds while doing away with the afore described economic and technical drawbacks.
Such an apparatus shall furthermore be fully reliable and easily implemented with the use of known, readily available techniques.
Such main aim of the present invention, along with further features of the present invention, is reached in a method to mold bottles of thermoplastic material through the use of a sensibly reduced number of preform molding molds in which the number of cavities are increased and the preforms produced in such cavities are removed therefrom in clusters that are conveyed to an appropriate blow-molding station in an orderly sequence and, therefore, according to a definite holding or waiting procedure.
In order to prevent the preforms from cooling down in a differentiated manner, and therefore in a manner that is not appropriate in view of a correct blow-molding process, during said holding period, the various preform clusters are cooled down, when still in the related mold cavities, in an intentionally differentiated manner aimed at ensuring that all preforms, when inserted in the blow-molding dies, exhibit the same, optimal temperature.